In a system having liquids or gasses that must be maintained within a predetermined pressure range, it is necessary to accurately measure the pressure. Numerous devices are available to measure pressure. Some devices indicate pressure while others act as transducers by converting the measured pressure to a calibrated quantity to be transmitted to another system. For example, in a system which utilizes electronic circuits to automatically control the pressure, it is desirable to provide an electrical representation of the pressure that can be provided as an input to the electronic circuitry. One such device that is used to generate an electrical representation of the pressure is a capacitance manometer, or capacitance diaphragm gauge.
An exemplary capacitance manometer is described in U.S. Pat. No. 3,557,621, issued on Jan. 26, 1971. Briefly, such a capacitance manometer has a diaphragm comprising an electrically conductive material that is supported along the periphery of the diaphragm by a housing or other such support structure. Typically, the housing is constructed from an electrically conductive metal so that the housing provides an electrically conductive path to the diaphragm. The diaphragm, which functions as one electrode of a capacitor, is positioned proximate to at least one reference electrode, which, as described in U.S. Pat. No. 3,557,621, is preferably fixed. For example, the fixed reference electrode is advantageously mounted on a ceramic disc substrate which is itself mounted to the housing.
One side of the diaphragm is exposed to a known or reference pressure and the other side of the diaphragm is exposed to an unknown, variable pressure that is to be measured. A differential in the pressure between the two sides of the diaphragm cause the center of the diaphragm to move in the direction of the lower of the two pressures and thus causes the diaphragm to develop a curvature. Such movement and resulting curvature causes the center of the diaphragm to move closer to or further from the fixed reference electrode of the capacitor and thus causes a corresponding change in the capacitance between the two electrodes. The capacitance between the two electrodes can be monitored, for example, by the circuit shown in FIG. 2 of U.S. Pat. No. 3,557,621, to thereby detect the movement of the diaphragm and thus detect changes in the pressure. The electrical output signal of the circuit can be measured by known devices and calculations performed on the measured signal value to provide an indication of the pressure differential.
The capacitance manometer described above is a high impedance device that operates with very small currents. Therefore, any leakage currents caused by moisture in the device, stray capacitances, and the like, can cause inaccuracies in the measurements. As set forth in U.S. Pat. No. 3,557,621, the fixed reference electrode of the variable capacitor is advantageously surrounded by a conductive guard ring that is typically concentric with the fixed reference electrode. A signal of substantially the same instantaneous voltage and phase as detected on the fixed reference electrode is applied to the conductive guard ring so as to effectively block leakage currents between the diaphragm (i.e., the movable electrode) and the fixed reference electrode. By blocking the leakage currents, the accuracy of the measurements obtained from the capacitance manometer is significantly increased, particularly with respect to the measurement of small pressure differentials wherein the movement of the diaphragm is relatively small. For example, capacitance manometers have been constructed that measure pressure differentials as small as approximately 10.sup.-9 atmospheres.
It has been found that the accuracy of a variable capacitance manometer, such as described in U.S. Pat. No. 3,557,621, can vary substantially in response to changes in the temperature to which the manometer is exposed. One such temperature effect is caused by the differential expansion and contraction of the fixed electrode support (e.g., a ceramic disc) on which the fixed reference electrode is supported with respect to the metallic housing of the manometer. For example, in exemplary capacitance manometers, the fixed electrode is mounted on a ceramic disc having a different coefficient of thermal expansion than the metallic housing. The differential expansion and contraction causes a relative movement of the fixed electrode support and the fixed reference electrode with respect to the metallic housing by causing the fixed electrode support and the fixed electrode to bend and develop a curvature. Thus, the center of the fixed reference electrode may move closer to or further away from the diaphragm as a result of a change in the temperature. The movement of the fixed reference electrode due to temperature causes a change in the capacitance that is indistinguishable from the change in capacitance due to changes in the capacitance caused by changes in pressure. Furthermore, it has been found that the effect of temperature changes are not predictable since the relative movement of the fixed electrode support with respect to the metallic housing typically does not repeat itself as the temperature increases and decreases. Thus, changes in the temperature cause spurious capacitance changes that affect the accuracy of the measurements. In order to prevent the temperature related changes in capacitance, it is desirable to reduce the relative movement of the fixed electrode support with respect to the metallic housing.
Another problem that continues to exist with respect to commercially available capacitance manometers is a small amount of residual stray capacitance. These stray capacitances exist for example between the fixed reference electrode and the metallic housing wherein the ceramic material of the fixed electrode support acts as a dielectric between the fixed reference electrode and the housing. Similar stray capacitances exist between the guard ring and the metallic housing. Although the guard ring substantially reduces the stray capacitances through the fixed electrode support to the metallic housing, heretofore, it has not been feasible to completely eliminate the stray capacitances. For example, the stray capacitances have been reduced to approximately 1 picofarad in one known commercially available capacitance manometer. The stray capacitances also vary in accordance with changes in temperature and thus introduce another uncontrolled variable into the measurements obtained using a capacitance manometer. Thus, it is desirable to further reduce the stray capacitances.